An active energy ray-curable resin composition can be cured by use of an active energy ray such as an ultraviolet ray or an electron beam. The active energy ray-curable resin composition is used in many industrial sectors as a surface coat agent, an adhesive, a pressure sensitive adhesive, a sealing agent, a paint, and the like in view of its transparency, quick-curability, fixability, and the like.
Since the active energy ray-curable resin composition is transparent or semi-transparent, it is difficult to visually determine a cure state of the active energy ray-curable resin composition or presence or absence of a quality abnormality in the active energy ray-curable resin composition. Especially the active energy ray-curable resin composition which is used as an adhesive and insufficiently cured causes many cases of complaints about products. Normally, it is necessary to test a quality of each of the products in terms of a cure degree of the active energy ray-curable resin composition. However, actually, there is no effective and realistic method for testing a product quality.
Therefore, in order to cure the active energy ray-curable resin composition, an excess active energy ray whose amount is not less than a sufficient amount is directed to the active energy ray-curable resin composition, so as to guarantee a cure state of the active energy ray-curable resin composition.
However, even active energy ray irradiation in a sufficient amount cannot determine whether or not a part to which the active energy ray-curable resin composition is applied is irradiated with the active energy ray. Further, it is impossible to detect an unsteady production state such as a deterioration in active energy ray irradiation lamp and/or defocusing of the active energy ray irradiation lamp. Therefore, a method for checking a curing reaction state of the active energy ray-curable resin composition is desired. A method has been known for checking a cure state of a resin composition by a Fourier transform infrared spectroscopy (hereinafter referred to as an FT-IR method) or a method for finding a hardness change (e.g., a Young's modulus) (see the description of “New Technology of UV/EB Curing and Application, p. 55, CMC Publishing CO., LTD.).
The FT-IR method is an analysis method in which an infrared absorbed amount in accordance with a specific functional group is measured by infrared irradiation and a conversion degree of the specific functional group is found by fluctuations in the infrared absorbed amount, so that a cure degree is measured.
In contrast, according to the method for finding a Young's modulus, it is possible to find a cure degree by a change in Young's modulus of a resin composition.
However, such check methods as described above are insufficient in accuracy and require time and cost for carrying out a test. In view of the circumstances, a method is required for more simply checking a curing reaction state of the active energy ray-curable resin composition.
Patent Literature 1 discloses a method for visually checking, by a colored degree of a leuco dye, whether or not an ultraviolet-curable epoxy resin is cured.
Patent Literature 2 discloses a curable pressure sensitive adhesive sheet in which it can be easily visually checked, by a color change, whether or not the curable pressure sensitive adhesive sheet is being cured without fail by light irradiation.
These methods are very simple methods which allow visually checking, by a color change, whether or not a resin is cured.
Patent Literature 3 discloses a method for producing a printed circuit board which is characterized by causing a photosensitive electrodeposition paint to contain a leuco dye. What is disclosed in Patent Literature 3 is as follows: A resist can develop color in a case where a photosensitive resist is formed by use of the photosensitive electrodeposition paint containing the leuco dye. This makes it possible to (i) visually check that the resist has been formed in accordance with a pattern mask and (ii) reduce a rejection rate in a manufacturing process without losing a photosensitivity of an electrodeposition resist.